As known in prior art, when a paper machine is started or after a web break, a tail of the web is passed through the paper machine by cutting from the web a narrow lead-in strip, which is guided manually through the machine by using air jets as well as different guide plates and threading devices. Continuously increasing running speeds of paper machines have caused increasing problems in threading of the web and thus new types of arrangements have been needed in order to accomplish threading of the web.
With respect to the prior art relating to the invention, reference is made to U.S. Pat. No. 3,355,349, which discloses a belt conveyor intended for transfer of a lead-in strip to a calender or to a reel-up, or a belt conveyor disposed before a calender. This known belt conveyor comprises two reversing rolls and a closed and air pervious belt loop disposed therebetween and having an upper run which is subjected to a vacuum. This vacuum is produced by means of a suction box which is placed inside the belt loop and which creates a vacuum effect on the upper run of the belt to keep the lead-in strip in contact with the conveyor belt. A drawback in this known device has been that the device which is based on a suction box is rather complex and heavy in structure and it includes a large number of wearing parts and takes much space. This known device lacks the possibility of profiling in a longitudinal direction, and in terms of servicing it is not advantageous. In this arrangement known from prior art, there is a high vacuum on the entire run with the result that there is created heavy friction, and thus large motors are required for conveying the conveyor belt and the web. The purpose of the present invention is to develop further the above-mentioned conveyor device so that the above-noted drawbacks may be avoided.
With respect to the prior art relating to the invention, reference is also made to FI Patent 69145, which discloses a device for conveying and guiding a lead-in strip of a web in a paper machine. This prior-art device comprises a conveyor belt arranged around two or more reversing rolls, which belt is pervious to air and has devices arranged within its loop for producing a vacuum effect on the run of the conveying belt on which the lead-in strip is conveyed, the lead-in strip being caused to adhere to and be held in contact with the run of the conveyor belt by means of the vacuum effect. On the conveying run of the conveyor belt, inside its loop, there are provided air blow means which include guide plates extending substantially parallel to the plane of the conveyor belt and the conveying run, in connection with which plates a dynamic vacuum effect can be produced by means of air blowings, the lead-in strip being caused to adhere to and be kept in contact with the conveying run of the conveyor belt by means of the vacuum effect. This known arrangement requires an external source of air and a rather large amount of air. This known device suffers from the problem that the air blow means placed one after the other in the running direction of the belt produce a wavelike vacuum curve, which changes from a negative pressure into a positive pressure just before the next air blow means. A problem in this kind of device is that it may cause the web to form bights at the areas with a positive pressure. The purpose of the invention is to develop further this known conveyor device such that the drawbacks described above may be avoided.